Google did a major update Penguin 2.0 which impacting 2.3% of queries now have to see who are affected and who are gaining.
Read more about Google Penguin 2.0 Update
What to expect in SEO in the coming months – Matt Cutt
Penguin 4, With Penguin 2.0 Generation Spam-Fighting, Is Now Live
Watch Out: Google’s Penguin 2.0 Is Fast Approaching
Penguin 2.0: PANIIIIIIIIC!!!…(or not)
Penguin 2.0 usually hits for unnatural backlinks here are some possible reasons of Penguin 2.0 hits
- Do you have too many links with exact match anchor text?
- Do you have too many sitewide/blogroll link?
- Do you have too many links from weak/low quality sites?
- Do you have too many links from own network sites?
- Do you have links from google penalized/de-indexed sites?
- Do you have links from malware sites?
- Do you have links from offtopic sites?
- Do you have Advertorial links?
- Do you have automated links?
- Do you have too many redirects links?
- Is your your link growth is unnatural ?
You can read a case study here.